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Always Tweaking the Design

Always Tweaking the Design. 1. 4. Endless (?) quest for perfection. The photogun works well, No anticipated near-term changes. 2. 3. 130 kV Inverted Gun. Higher Voltage = Better Transmission = Better Beam Quality (and maybe longer lifetime).

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Always Tweaking the Design

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  1. Always Tweaking the Design 1 4 Endless (?) quest for perfection The photogun works well, No anticipated near-term changes 2 3

  2. 130 kV Inverted Gun Higher Voltage = Better Transmission = Better Beam Quality (and maybe longer lifetime) Prebuncher operating at modest power takes care of the rest

  3. Improve Lifetime with Large Laser Spot? (Best Solution – Improve Vacuum, but not easy) Bigger laser spot, same # electrons, same # ions Ionized residual gas strikes photocathode Ion damage distributed over larger area

  4. Enhanced lifetime for Qweak Increase size of laser beam from ~ 0.35 mm to ~ 0.7 mm dia. Can we use ~ cm size laser beams? Not in today’s CEBAF photogun. How far can we extrapolate? Need a better cathode/anode optic “Charge and fluence lifetime measurements of a DC high voltage GaAsphotogun at high average current.,” J. Grames, R. Suleiman, et al., Phys. Rev. ST Accel. Beams 14, 043501 (2011)

  5. 4pElectron Spin Manipulation Horizontal Wien Filter Two Solenoids Vertical Wien Filter • Harder to flip spin than we imagined, more “things” change than just spin direction • Beam orbit downstream of HWien sensitive to laser/prebuncher (mis-)phasing • Don’t know what to do about this….

  6. Electron Spin Reversal for PV “Spin Reversal” Vertical Wien = 90 deg Two Solenoids = ±90 deg “Longitudinal Polarization” Horizontal Wien = {-90…+90} From Gun Pondering a new beamline, but will it behave differently? Reluctant to move prebuncher. Modification would provide an opportunity to improve beamline vacuum…. LEFT RIGHT

  7. Two Wien Improvements • Do a better job characterizing optics, and compensate asymmetric focusing using quads, to achieve “perfect” spin flip • Mis-matched E and B fields create mini-chicane, with e-beam displaced from zero potential, which introduces a time delay…

  8. All of the same knobs exist today…. Rotatable GaAs Photocathode Attenuator IA LP HWP LP PC WP LP Shutter Helicity Fiber Helicity Generator 15° Dipole HV Supply (0 – 90 V) nHelicity Fiber Vacuum Window Delayed Helicity Fiber T-Settle Fiber Hall RHWP Charge Feedback (IA) V-Wien Filter Spin Solenoids Parity DAQ Target H-Wien Filter BCM Charge Feedback (PITA) BPMs Position Feedback Pockels Cell HV Supply (0 – 4 kV) IHWP 5 MeV Helicity Magnets CEBAF PZT Mirror Electron Beam

  9. Stewart Platform: complete RC of PC alignment LINUX-based software/freeware from hobbyists who build flight simulators X, Y, pitch, roll and yaw

  10. Initial attempts and problems encountered This approach needed big capacitors…. lots of current Two commercial high-speed / high-voltage transistor (~$8000) Exaggeration of voltage droop on cell and subsequent re-charge after a helicity flip. Droop causes a serious problem when helicity flip rate was changed from a toggle to a pseudo-random pattern: pockels cell “memory” high-voltage switch (~$10,000) All-in-one commercial bipolar Charge droop was greatly improved, but high speed ringing of the cell was a problem for the settling time. In addition, the large high-speed switching currents created a noise induced helicity pickup on sensitive helicity DAQ components. Work of John Hansknecht

  11. Solution: Opto-diode • I = CΔV/ Charge time • +/- quarter wave voltage = 5120V • Cell capacitance = 6pf • Desired charge time = 100us • Calculated current is only 307uA !! Encapsulated Opto-diode $67 each Work of John Hansknecht

  12. Pockels cell λ/2 flipping at 1kHz. Perfect symmetry and no voltage droop over time. New Opto Diode Pockels Cell Switch Push-Pull Circuit Pockels cell λ/2 transition optical result. ~70us with no ringing. 1 2 3 The old switch was limited to 30 Hz helicity flip rates due to power handling limitations. Now we flip at 1kHz for ~ 400$. Pockels cell “memory” not an issue anymore…. 4 Work of John Hansknecht

  13. MottPolarimeter at CEBAF

  14. M. Steigerwald first reported MeV Mott Polarimeter at SPIN 2000, with accuracy 1.1% • Still that accurate? Can it be more accurate ??? => motivates a new “campaign” to study 5 MeV Mott

  15. Accurate Mott Polarimetry • List of improvements and tasks: • Reduce background using time of flight analysis, laser at 31 MHz • Reduce background using Be dump plate • Upgrade DAQ to operate at higher data rates, higher beam currents (i.e., measure polarization at your current) • Exhaustive studies of systematic errors • GEANT modeling and assistance from theorists to help pin down the Sherman function at zero foil thickness • Different Zs, different energies • Planned Summer 2014 tests using 1/4CM at 4K….. M. McHugh, Joe Grames, Riad Suleiman

  16. Role of Beam Dump Background 12 ns round trip time 499 MHz (2ns) Gold 0.1 um Gold 1.0 um Gold 10 um M. McHugh Joe Grames Riad Suleiman

  17. Dipole Suppression Dipole deflects primary and secondary electrons +5 A OFF

  18. Time Of Flight Separation fbeam = 31.1875MHz 16 ns repetition rate > 12 ns “clearing time” TARGET STRAGGLERS DUMP TARGET STRAGGLERS DUMP

  19. Be Dump Design Kalrez™ high temp (240C) o-ring Cu Al Number of Electrons C Dump should work fine to 1 kW (200uA @ 5MeV) so limitation will be deadtime. Be Total Momentum [MeV/c] M. McHugh, Joe Grames, Riad Suleiman

  20. TMONO Harmonic Resonant Cavity Bunched electron beam can be described as Fourier series…. I(t) = a0 + a1cos (wot + q1) + a2cos (2wot + q2) + a3 cos(3wot + q3) + … …..a cavity sensitive to each mode = bunchlength monitor (Brock Roberts)

  21. On the bench “antenna/radiator” placed where the e-beam would travel. The antenna was driven with a 20 dBm,1556 MHz signal through a step recovery diode. Yes, cavity resonates at many harmonic modes.

  22. Installed at ITS With beam delivered to dump, spectrum analysis shows harmonics visible to 18.396GHz, the 12th harmonic of 1533MHz (measurement made at 150kV, 25uA, superlattice cathode)

  23. Bandwidth determines Resolution

  24. At CEBAF 500 kV spectrometer line Laser optical pulse (a) (b) O-scope Brock cavity (c) Brock Roberts will use RF model of cavity to see if it is sensitive to transverse beam motion and beam size variation…. helicity correlated beam size monitor? Lower noise floor for current monitoring? Electron bunch 500 kV spectrometer line

  25. Load-locked photogun and baked beamline: Pressure ~ 4 e-12Torr

  26. 350kV Inverted Gun 200kV Inverted Gun • Longer insulator • Spherical electrode • Thin NEG sheet to move ground plane further away

  27. To do list • Stewart Platform for Pockels cell: complete remote control of all alignment parameters • 2 kHz Pockels cell switch, stable voltage and minimum transition time between helicity states • Improve 2-Wien Spin Flipper optics • Beam envelope matching to maximize adiabatic damping • Accurate Mott measurements at Injector • Re-wire 5MeV helicity magnets to provide HC focusing, and evaluate experimental sensitivity to HC spot size variation • Use POISSON and genetic algorithm to improve the field uniformity of pockel cell • Helicity correlated beam size monitor? • Beam current monitor with low noise floor?

  28. Benchmarking PARMELA Simulation Results Against Beam-Based Measurements at CEBAF/Jefferson Lab – work of AshwiniJayaprakash, JLab PARMELA Simulation Results Measurements at CEBAF/JLab Similar Trends Message: Beam quality, including transmission, improves at higher gun voltage

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